The present invention relates to non-damaging drilling fluids, essentially solid-free, whose main constituents are (a) a biopolymer of the polysaccharide type, (b) a filtrate reducer essentially consisting of starch, and its derivatives, in presence of cellulose microfibers.
The exploitation of complex, fractured and/or depleted production zones, and the application of new drilling techniques such as open-hole, slim-hole drilling etc., require the development of new drilling fluids (Drill-In-Fluids) which do not damage the reservoirs in order not to reduce the productive capacity of the wells. For this purpose, the polymeric additives and possible particles contained in the fluid and also those produced as a result of the drilling, must be prevented from penetrating into the formation and thus irreversibly blocking the rock pores and jeopardizing the productivity. Drill-In-Fluids prevent damage to the reservoir by the formation on its surface of a thin filter cake which is impermeable and can be easily removed when the well is put into production or by the action of enzymes or acids.
Various formulations have been studied over the years, that are capable of producing a filter cake which sticks firmly to the well walls and is sufficiently thin as to avoid operative problems, such as xe2x80x9cstuck pipexe2x80x9d. Furthermore the permeability of the filter cake must be so reduced as to minimize fluid loss due to filtration of the drilling fluid through the rock formation.
Patent literature describes numerous formulations capable of solving the above problems.
For example, EP-A-691,454 describes water-based drilling fluids, whose main components are a non-ionic biopolymer of the polysaccharide type, a polymerized starch and calcium carbonate particles having a specific particle-size distribution.
The composition described in EP-A-691,454 however has the disadvantage that the calcium carbonate returns to the surface together with the cuttings, thus requiring a further addition of the carbonate particles to the fluid which is re-circulated. The filter cake formed by calcium carbonate, moreover, is too thick.
A drilling fluid composition has now been found, which overcomes the drawbacks specified above.
In accordance with this, the present invention relates to a water-based drilling fluid comprising:
(a) water,
(b) at least one biopolymer of the polysaccharide type,
(c) at least one filtrate reducer selected from (c1) starches with different cross-linking degrees in presence of cellulose microfibers (c2),
(d) optionally a hydrosoluble polymeric surface-active agent having an HLB ranging from 10 to 16, preferably from 11 to 14.
In the preferred embodiment, the biopolymer (b) is present in the drilling fluid in a concentration ranging from 2 to 10 grams/liter, preferably from 4 to 7 grams/liter; the cross-linked starch (c1) in a concentration of 10 to 30 grams/liter, preferably from 12 to 20 grams/liter; the cellulose microfibers in a quantity ranging from 10 to 30 grams/liter, preferably from 12 to 20 grams/liter.
The biopolymers of the polysaccharide type (b) are hydrosoluble polymers produced by bacteria or fungi. Their function is to increase the viscosity of the fluid in order to keep the filtrate reducers and possible drilled solids suspended both during the drilling period and during possible stop in the fluid circulation. They may be of the ionic or non-ionic type. Examples of non-ionic biopolymers are scleroglucan and welan gum. A commercial product belonging to this group is Actigum(copyright) CS 6, produced by SKW Trostberg, described as a branched homopolysaccharide produced by a fungus of the Sclerotium type by means of an aerobic fermentation process. The main chain of scleroglucan consists of residues of the xcex2-D-glucopyranosyl type with bonds (1-3) whereas the branchings consist of D-glucopyranosyl residues with xcex2 bonds (1-6) every three glucoside groups.
Another non-ionic biopolymer is welan gum produced by the Alcaligenes species. This is a heteropolysaccharide similar to scleroglucan but with residues of the xcex1-L-rhamnopyranosyl or xcex1-L-mannopyranosyl type.
A further example of the biopolymer (b) is xanthan gum. This is an anionic branched heteropolysaccharide produced by the bacterium Xanthomonas campestris, characterized by a main chain consisting of D-glucopyranoside residues with lateral chains consisting of piruvate and acetate groups.
Another main component of the composition of the present invention is the filtrate reducer (c), selected from starches and their derivatives (c1). The above starches are cross-linked by means of bifunctional reagents such as phosphates or epichlorohydrin, and are therefore insoluble in water. The starches however are compatible with the biopolymers (b) and their purpose is to form a homogeneous and impermeable filter cake on the rock surface. A non-limiting example of cross-linked starch which is commercially available is that produced by Chemstar(copyright).
Another component of the filtrate reducer consists of cross-linked colloidal cellulose fibers (c2), produced by micro-organisms, of the Acetobacter type, under aerobic conditions. These products are insoluble in water and have a high surface area with a behaviour in aqueous phase which is totally insensitive to high temperatures, pH, salinity and shear. The preparation of these cellulose microfibers is known and described in various patent documents, for example U.S. Pat. No. 5,079,162 and U.S. Pat. No. 5,144,021. Combined with starch and biopolymer, the cellulose fibers enable the formation of a filter cake having an extremely low permeability and which can be easily removed by the simple put into production of the well.
An optional component of the composition of the present invention (d) is a hydrosoluble polymeric surface-active agent having an HLB ranging from 10 to 16, preferably from 11 to 14. A non-limiting example of a commercially available non-ionic surface-active agent consists of alkyl-polyglucosides.
As is known to experts in the field, the composition of the present invention may contain other additives, such as biocides, lubricants, clay inhibitors, pH regulators.
The density of the drilling fluid, moreover, may be increased by the addition of saturated solutions of sodium, potassium, calcium chloride, or sodium, potassium or cesium formates.
The water-based drilling fluid of the present invention is substantially solid-free and has various advantages with respect to fluids containing solids. In fact:
1) the viscosity of the drilling fluid of the present invention at high shear values is sufficiently low to guarantee low pressure drops during drilling, above all in the case of slim-hole drilling (i.e. a small drilling diameter often used to reduce costs);
2) the gel and viscosity values at low shear values are sufficiently high to keep the cuttings in suspension when the fluid circulation is stopped, thus avoiding the formation of deposits;
3) the absence of calcium carbonate particles allows an easier recycling of the drilling fluid after passing through the surface equipment (shale shaker) to recover the cuttings; in fact, it is not necessary to reintegrate the calcium carbonate;
4) the formulation of the present invention is particularly suitable for formations with a low/medium permeability (up to 500 mD) and does not require an accurate knowledge of the formation permeability, as is the case, on the contrary, for fluids which use solids, whose particle-size distribution greatly depends on the reservoir permeability.
The drilling of the non-productive zone of the well can be effected initially also using the conventional drilling fluids containing bentonite and/or barite. Near the productive zone, these fluids are then substituted with those of the present invention, which are recirculated in order to remove the conventional fluid, in particular bentonite or barite or other potentially harmful additives. At this point the drilling of the productive zone can be restarted with circulation of the above fluid, thus avoiding damage to the reservoir.
The composition of the drilling fluid of the present invention is prepared by the simple mixing of the components. It is preferable, however, for the mixing to be carried out by the successive addition of the single components, care being taken that each single component is completely mixed before the addition of the subsequent one.
A further object of the present invention relates to a process for drilling oil and/or gas wells by means of water-based drilling fluids, characterized in that the drilling fluid comprises:
(a) water,
(b) at least one biopolymer of the polysaccharide type,
(c) at least one filtrate reducer selected from (c1) starches with different cross-linking degrees in presence of cellulose microfibers (c2),
(e) optionally a hydrosoluble polymeric surface-active agent having an HLB ranging from 10 to 16, preferably from 11 to 14.